Field of the Invention
The present invention relates to an image capturing apparatus capable of obtaining distance information for each pixel.
Description of the Related Art
Recently, a solid-state image sensor not only performing capturing but also having various functions added has been proposed.
As one of the functions added to the solid-state image sensor, the function of obtaining a distance image using a TOF (Time-of-Flight) method has received attention. The TOF method derives a distance to a target object by projecting pulsed light from a light source to the target object and then measuring a time (the delay time of pulsed light) that has elapsed before reflected light from the target object reaches an observation point. It is known that the speed of light is 3×108 [m/S]. Therefore, for example, if the delay time is 2 [nS] when the observation point and the light source are placed at the same point, it is obvious that the total flight distance of light is 60 [cm] and the distance to the target object is 30 [cm] which is half the total flight distance. The solid-state image sensor capable of obtaining the distance image by making use of the principle of this TOF method to obtain distance information to the object for each pixel has been proposed.
Japanese Patent Laid-Open No. 2004-294420 discloses a solid-state image sensor including a plurality of charge accumulation portions which accumulate signal charges from one light receiving element and a transfer switch which controls charge transfer from the light receiving element with respect to each charge accumulation portion. In such a solid-state image sensor, the transfer switches corresponding to the two charge accumulation portions are controlled alternately to distribute charges generated in the light receiving element upon receiving reflected pulsed light at the two charge accumulation portions. Then, the distance information for each pixel can be obtained by obtaining the ratio of the charges of the two charge accumulation portions.
Further, Japanese Patent Laid-Open No. 2010-213231 discloses a solid-state image sensor capable of obtaining distance information in the arrangement having one transfer switch and one charge accumulation portion per pixel by distributing charges not to each pixel but to two pixel groups, for example, pixels in an even numbered row and an odd numbered row, respectively.
In the conventional technique disclosed in Japanese Patent Laid-Open No. 2004-294420 or Japanese Patent Laid-Open No. 2010-213231, two separate transfer switches are driven to distribute charges to the plurality of accumulation portions. It is therefore necessary to match the driving timings of the two switches accurately in order to distribute the charges accurately.
However, if the threshold variations, shifts in the transfer pulse, or the like of the transfer switches are generated, an error occurs in a timing for switching a charge distribution destination. As a result, accurate signal charges cannot be obtained and wrong distance information may be derived.